Boss Construction For Air Bleeders, Drain & Temperature Sensor Bosses were made from 1/8 inch thick aluminum. The center hole was taped out to #6-32 Thd and the rivet holes were drilled 3/32 inch OD. A center locating hole was marked on the tank surface and drilled to clear the screw thread. Then the boss with nylon screw threaded in so that the free end entered the hole in the tank. Then the first rivet hole was drilled into the tank. To maintain proper position a rivet was inserted into the hole, and the following two holes were drilled. Boss position relative to the tank was marked so that when the whole works was removed, the boss could be in exactly the same position. JB Weld was used to seal the boss to the tank surface. The rivet ends were covered with JB Weld before inserting them into place. The nylon screw was used keep the JB Weld from blocking the bleed hole. After pulling the rivets and the JB Weld had time to stiffen, the excess material was removed, and the nylon screw was removed. Later the mandrel rivet holes were sealed with JB Weld. Coolant drain and air bleed screws use "O" rings for sealing. To ensure the "O" rings stay centered, they are glued to the bottom side of the screw head with JB Weld.